This method is defined in USNRC Regulatory Guide 1.92. This combination method is the most technically correct for earthquake loads, because it attempts to estimate the actual intermodal correlation coefficient based upon empirical data. The total system response is calculated as:
Where:
Rs = the peak value of the response of the element due to mode s
eks = intermodal correlation coefficient = [ 1 + {( wk' - ws') /(ßk' wk + ßs' ws)}2 ]-1
wk' = wk [ 1 - ßk2 ]1/2
ws' = ws [ 1 - ßs2 ]1/2
ßk' = ßk + 2 / ( td wk )
ßs' = ßs + 2 / ( td ws )
wk = frequency of mode k, rad/sec
ws = frequency of mode s, rad/sec
ßk = ratio of damping to critical damping of mode k, dimensionless
ßs = ratio of damping to critical damping of mode s, dimensionless
td = duration of earthquake, sec
The load duration (td) and the damping ratio (ß) can be specified by using the Load Duration (DSRSS) (sec) and Damping (DSRSS) (ratio of critical) parameters.